The Arabidopsis root stele transporter NPF2.3 contributes to nitrate translocation to shoots under salt stress

Summary In most plants, NO3- constitutes the major source of nitrogen, and its assimilation into amino acids is mainly achieved in shoots. Furthermore, recent reports have revealed that reduction of NO3- translocation from roots to shoots is involved in plant acclimation to abiotic stress. NPF2.3, a member of the NAXT (nitrate excretion transporter) sub-group of the NRT1/PTR family (NPF) from Arabidopsis, is expressed in root pericycle cells, where it is targeted to the plasma membrane. Transport assays using NPF2.3-enriched Lactococcus lactis membranes showed that this protein is endowed with NO3- transport activity, displaying a strong selectivity for NO3- against Cl-. In response to salt stress, NO3- translocation to shoots is reduced, at least partly because expression of the root stele NO3- transporter gene NPF7.3 is decreased. In contrast, NPF2.3 expression was maintained under these conditions. A loss-of-function mutation in NPF2.3 resulted in decreased root-to-shoot NO3- translocation and reduced shoot NO3- content in plants grown under salt stress. Also, the mutant displayed impaired shoot biomass production when plants were grown under mild salt stress. These mutant phenotypes were dependent on the presence of Na+ in the external medium. Our data indicate that NPF2.3 is a constitutively expressed transporter whose contribution to NO3- translocation to the shoots is quantitatively and physiologically significant under salinity. Significance Statement The constitutively expressed root stele transporter NPF2.3 from Arabidopsis promotes nitrate loading into the xylem sap and its contribution to nitrate allocation to shoots becomes quantitatively significant upon salt stress, which was unexpected because salinity is known to depress nitrate translocation. The loss-of-function mutation npf2.3 resulted in reduced biomass production when plants were grown in soil supplemented with sodium concentrations compatible with life cycle completion, indicating that NPF2.3 contributes to plant acclimation to moderate salinity